Pulsed lasers are widely used for precision energy delivery in equipment ranging from surgical devices to lithography systems for forming electronic microcircuits. Among various types of pulsed lasers, the excimer laser, a type of electric discharge gas laser, has been widely used for microcircuit lithography. In lithography, the surface of a wafer is exposed to a pattern of high energy, typically in the high-energy ultraviolet (UV) range, that forms the various microcircuit components. A photoresist or other material deposited on the substrate surface responds to the high-energy pattern, projected onto the surface using a precision optical system, forming the various circuit features in a combination of exposure and etching stages.
One well-known difficulty that is inherent to the microlithography process relates to the high peak-power density that is required and its impact on optical components. The optical subsystems that condition and deliver high-energy pulsed UV or other radiation, typically from excimer or other lasers, can be damaged over time and the lens assemblies used can be extremely costly to replace. Even the down time for repair or replacement of the supporting optical components is highly undesirable, slowing productivity in the wafer-to-microcircuit fabrication process.
One practice for reducing the effects of high peak-power density for pulsed laser sources is to smooth or modify the laser pulse before directing it through the optical system. Pulse stretchers for changing the distribution of pulse energy are described, for example, in U.S. Pat. No. 6,928,093 entitled “Long Delay and High TIS Pulse Stretcher” to Webb et al. and U.S. Pat. No. 6,535,531 entitled “Gas Discharge Laser with Pulse Multiplier” to Smith et al. As an illustrative example of pulse stretching, a plot showing a single laser pulse over time is shown as a pulse 18 in FIG. 1A. FIG. 1B shows how a pulse stretcher can modify the distribution of pulse energy over time for forming a conditioned pulse 28. The value Tis or “time-integrated square” is a metric used to express the effective pulse duration of the stretched pulse, as described, for example, in the '093 Webb et al. disclosure. One aspect of pulse stretching is delay of some or all of the initial laser pulse, as shown for pulse 28 in the example of FIG. 1C.
Pulse stretching generally uses a beamsplitter that redirects a percentage of incident light, causing the “ringing” effect of the pulse pattern as its amplitude decays over time, as shown in the example of FIG. 1B. Using mirrors or other highly reflective surfaces, delay can also be used to provide pulse stretching with a discrete number of pulse components, as is shown in a simple case with two components in modified pulse 28 of FIG. 1E.
Pulse stretching and delay techniques not only reduce the peak power of the laser, but also help to minimize coherence of the laser light. Without pulse stretching or other measures, highly coherent lasers can create undesirable interference artifacts in the laser illumination, commonly known as speckle.
Although pulse delay and stretching techniques have been practiced, however, conventional solutions for changing the pulse profile in these ways require complex arrangements of components and necessitate considerable alignment efforts in order to provide the laser beam along a single beam path without increased beam width. For example, four or more facing curved mirrors must be adjusted to align the optical paths in solutions shown in both the '093 Webb et al. and '531 Smith et al. disclosures.
Another problem that is not addressed by conventional pulse delay or stretching solutions relates to the need for improving pulse-to-pulse energy uniformity. One undesirable characteristic of the excimer laser is that the power level, pulse-to-pulse, can vary over a range. Conventional pulse stretching or delay solutions can smooth out the pulse energy distribution, but do not offer ways for attenuating pulse energy at the same time. Pulse attenuation must be performed elsewhere in the optical system, either preceding or following the pulse stretching or delay apparatus. It would be advantageous not only to modify the energy distribution of the laser beam over time, but also to be able to modulate one or more portions of this energy distribution at the same time.